Chlorination of cellulose pulp

ABSTRACT

A process is provided for delignifying cellulose pulps that have previously been subjected to treatments with chlorine, or chlorine dioxide, or a mixture thereof, and with alkali, and is intended to be carried out as a third or subsequent stage in the multi-stage bleaching of cellulose pulp, which comprises delignifying the cellulose pulp with a mixture of chlorine and chlorine dioxide comprising from about 10 to about 50 percent chlorine dioxide calculated as available chlorine, at a pH within the range from about 1 to about 3.5 and at a pulp consistency within the range from about 3 to about 15 percent, and a temperature within the range from about 40* to about 70* C., for a time within the range from about 30 to about 300 minutes.

Croon et a1.

Apr. 9, 1974 CHLORINATION OF CELLULOSE PULP [75] Inventors: IngemarLiss-Albin Croon,

Salgvagen; Sten Oskar Dille'n, Strandvagen, both of Sweden [73]Assignee: M och Domsjo Aktiebolag,

Ornskoldsvik, Sweden [22] Filed: Apr. 8, 1 971 [21] Appl. No.: 132,367

Related US. Application Data [63] Continuation-in-part of Ser. No,774,171, Nov. 7,

1968, abandoned.

[52] US. Cl. 162/89, 8/108 [51] Int. CL, D21c 9/12 [58] Field of Search162/89, 88, 87; 8/108 [56] References Cited UNITED STATES PATENTS3,560,330 2/1971 Ahlborg 162/89 X 3,433,702 3/1969 Jack et a1 162/88 X3,652,388 3/1972 Croon et al.... 162/89 X 3,536,577 10/1970 Rapson162/89 X 2,166,330 7/1939 Vincent 162/87 X OTHER PUBLICATIONS Rydholm,Pulping Processes, 1965, p. 839. Scott Paper Company Handbook, 1st ed.,1956, p. 95.

Primary E.\'aminerS. Leon Bashore Assistant Examiner-Arthur L. Corbin [57 ABSTRACT A process is provided for delignifying cellulose pulps thathave previously been subjected to treatments with chlorine, or chlorinedioxide, or a mixture thereof, and with alkali, and is intended to becarried out as a third or subsequent stage in the multi-stage bleachingof cellulose pulp, which comprises delignifying the cellulose pulp witha mixture of chlorine and chlorine dioxide comprising from about 10 toabout 50 percent chlorine dioxide calculated as available chlorine, at apH within the range from about 1 to about 3.5 and at a pulp consistencywithin the range from about 3 to about percent, and a temperature withinthe range from about to about C., for a time within the range from about30 to about 300 minutes.

7 Claims, N0 Drawings This application is a continuation-in-part of Ser.No. 774,171, filed Nov. 7, 1968, and now abandoned.

This invention relates to a process for delignifying Chlorine isconsidered to be the most effective and least expensive of thedelignifying agents that are available. However, it has a tendency todecompose the cellulose, and therefore it is necessary to keep theamount Up to 130 C. in hot alkali refining.

cellulose pulp that has previously been subjected to 5 Of chlorine atthe minimum that give effective detreatments with chlorine, or chlorinedioxide, or a mixhghificahoh- The chlorination thus is usually carried tth f, d th ith lk li; d more i out in such a manner that there is asmall excess of l l t a ltit process f bl hi ll l chlorine present atthe conclusion of the chlorination. pulp which includes as a third orlater treatment stage, This excess is rather important to control. Ifthere is too following treatment with chlorine, or chlorine dioxide,high an excess remaining, the tendency of the or mixture th f d ialkali, a d li ifi ti chlorination stage to decompose cellulose isincreased, stage with a mixture of chlorine and chlorine dioxide at andif there is no excess, if the eXCeSS is $00 Small, a temperature i hithe range f about 4 t about the delignification of the cellulose isincomplete. If the 70 C, fo a i i hi the range f about 30 tochlorination must be applied in two stages, the risk of about 200minutes and at a pulp consistency within the decomposition of thecellulose is increased in the secrahge f about 3 percent to about 15percent, and stage, and therefore the arrangement of the treat- Th bl hif ll l l b i d b either ment stages is usually such that thechlorination is rethe sulfate or sulfite process is a multi-stageprocedure, Stricted to one Stage, and a Substitute hypochlorite 0F andusually includes as separate steps in any of a larger Chlorine dioxidestage is used instead, if Possiblenumber of sequences of one or more ofeach, The Chlorination, Stage is normally Performed a, a hl i i Stage,with a relatively low pulp consistency, from about 1 to b. an alkalitreatment or alkali extraction stage, about 4 Percent, and at lowtemperatures, from about c. a hypochlorite treatment stage, and 5 toabout C., for short treatment times, from about d. a chlorine dioxidetreatm t t 1/3 to about 2 hours. The quantity of chlorine used var-These treatments can be carried out in almost any or- 25 ieS With theproperties of the unbleached P p. but is der, and two or more of suchstages are frequently insufficient n y to g e n XCeSS Within the rangefrom eluded, but usually not in succession, but interleaved about 0.1 toabout 0.5 percent chlorine, based on the with one 01' more f th oth stweight of the dry pulp, at the conclusion of the treat- In general,sulfate pulps require more treatment menistages for complete bleachingthan sulfite pulps. Also, Chlorine dioxide can also be used in place ofchlorine in the case of sulfate pulps, chlorination, alkali treatin eChlorination g in this event, the amount of ment and chlorine dioxidetreatment are almost always chlorine dioxide used is sufficient tosupply an amount necessary while the hypochlorite treatment can someofchlorine within the range set out above sufficient to times be omitted,whereas in the case of sulfite pulps, ensure an excess of from 0.1 to0.5 percent chlorine at either the hypochlorite or the chlorine dioxidetreatthe end of the treatment. ments can be omitted, and the bleachingcan be re- The alkali extraction, stage E, is designed to remove ducedto as little as three or four stages. However, all the lignins that havebeen made alkali-soluble in the multi-stage bleaching processes employ achlorination chlorination stage, and is carried out at a relatively highstage and an alkali treatment stage, and the reason is pulp consistency.This is normally at from about 4 perthat these stages are required fordelignification. Some cent to about 18 percent; when using presses fordewasulfate pulps are more difficult to delignify than others, tering,up to 30 percent. The temperature can be rather and in the case of thesepulps, one chlorination stage low, from 25 to 30 C., but in theso-called hot alkali may be insufficient to delignify the pulpsufficiently. refining processes, for producing pulp-withahigh alphaHence, a complementary delignification stage can be cellulose content,temperatures of up to 130C. can be carried out in an additionalchlorination stage, or in a employed. The treatment time can be veryshort, for hypochlorite treatment stage, or in a chlorine dioxide justseveral minutes, up to from 4 to 5 hours. The treatment stage. amount ofalkali charged to the system varies according The conditions under whichthe various treatment to the properties of the pulp which it is desiredto prostages are carried out in conventional bleaching procduce. In thecase of paper pulps, it is usually carried out esses are summarized inTable A, which follows: in such a manner that the pH at the conclusionof the TABLE A i Pulp 4 consist- Tempera- BHGY, 1TB Stage Treatmentpercent C.) Time Amount 0 Chlorination 14 5-25 %-2 hours... Enough C1;or C102 to provide an end of treatment 0.1 to 0.5% excess C12 based on(Cl: or C10: pulp weight. E Al lt aii di r aetion 4-18 25-80 Severalmin- Enough NaOH to give a pH 8-12.51 at end of treatment.

(NaOH) ggelsrgo 4-5 H l iii) H hm vi iii, fihhiilfif2.ififiliifiiifiieSffirifiiifiiob"zi'i hiiiiififfioii t ifi fi r pH 8.5, usually10-11. 1 Ca 001): excess alkali D Gi le? dioxide 4-15 50-90 2-5 hoursEnough to provide 0.5 to 5% 01:, based on pulp weight, with an excess offrom 0.1

(C102). to 0.5% C 2 at end of treatment.

treatment is within the range from about 8 to about l2.5, generallyabout ll.

The hypochlorite treatment, stage H, is also carried out at relativelyhigh pulp consistencies, normally from about 4 percent to about 18percent. The temperatures are usually rather higher than in thechlorination, and range from about 25 to about 60 C. The treatment timeis also somewhat longer, from 1 hour and upwards, normally from two tofour hours. The quantity of hypochlorite charged to the system varieswith the type of pulp and the conditions, and is based on the amount ofchlorine provided. The amount can range from very small amounts, of theorder of 0.2 percent C1 by weight of the pulp, to 3% C1 based on theweight of dry pulp. This treatment is also carried out in a manner toensure that a small excess of chlorine remains at theconclusion of thetreatment, about 0.1 percent by weight of the pulp.

l-Iypochlorite having a pH at about and immediately below the neutralpoint of 7 strongly decomposes cellulose, and consequently an amount ofalkali is also charged to the system to provide a surplus of alkali, andmaintain the pH well above 7. The amount of alkali charged as NaOH canbe within the range from about 10 percent to about 100 percent by weightof the chlorine charged to the system in this stage, so as to ensurethat at the conclusion of the hypochlorite treatment the pH is above8.5, and is normally within the range from 10 to 11.

The chlorine dioxide treatment is carried out with the same pulpconsistency as the hypochlorite treatment, although in practice asomewhat lower consistency can be used, since the chlorine dioxide isobtained in a more dilute form than the hypochlorite solution. Thus, thepulp consistency during chlorine dioxide treatment can be within therange from about 4 percent to about percent. The temperature is higher,within the range from about 50 to about 90 C., and normally about 70 C.The treatment time is at least 2 hours, and can be from 3 to 5 hours.

The quantity of chlorine dioxide charged to the system is based on theamount of chlorine equivalent to the chlorine dioxide within the rangefrom about 0.5 to about 3% C1 based on the weight of the dry pulp, andsometimes even more than 3 percent, up to about 5 percent. Thistreatment is also carried out in a manner to ensure a small excess,about 0.l to 0.5% Cl at the end of the treatment.

The usual bleaching process for sulfate pulps includes six stages, inthe order C E H D E D, the letters corresponding to the treatment stagesset out above. An alkali extraction normally follows one or more stagesof delignification, to remove solubilized lignins DED DED D mzmrn muomUri-1::

Sulfite pulps can be bleached with shorter bleaching sequences, andillustrative sequences include O0 000 mm rnmrn :2: 03:0

It will be noted that in all of the sequences in common use for sulfateand sulfite pulps, the first treatment stage is invariably achlorination stage, and the second treatment stage is almost invariablean alkali or extraction stage, to remove the lignins and otheralkalisoluble extractives. In the single exception noted above, thehypochlorite stage serves as a substitute second chlorination stage,seeking to reduce the risk of cellulose decomposition in the firstchlorination stage. Anyhow as pointed out above also this hypochloritestage comprises an alkali treatment immediately following thechlorination. An alkali extraction is interposed wherever morealkali-soluble extractives are to be removed.

The difficulty of control of the chlorination stage so as to preventdecomposition of the cellulose has drawn the attention of numerousworkers in the pulp bleaching art, and many suggestions have been made.The most effective and least expensive alternatives, however, haveinvolved the substitution of a hypochlorite or chlorine dioxide stagefor a second chlorination stage, and a reduction in the severity of thefirst chlorination. Even in this variation, however, it is not possibleto prevent decomposition of the cellulose altogether, particularly whenthe complementary stage is a hypochlorite treatment.

It has recently been proposed that the chlorination be carried out usinga mixture of chlorine and chlorine dioxide. The amount of chlorinedioxide can be rather small, up to a major proportion. The chlorinationis otherwise carried out in the usual way under the treatment conditionsas set out in Table A above. However, the degree of ameliorationobtained of the tendency to decompose the cellulose is not under theseconditions sufiicient to compensate for the considerable increase in thecost of the chlorination, due to the higher cost of chlorine dioxide ascompared to chlorine. Moreover, the amelioration of the decomposition ofthe cellulose may also be accompanied by an incomplete delignification,as a result of which it becomes necessary to introduce anotherchlorination stage, or a hypochlorite or chlorine dioxide treatmentstage, so as to complete the necessary delignification In this event,there remains the possibility of decomposition of the cellulose duringthis complementary stage.

Ahlborg, US. Pat. No. 3,560,330, patented Feb. 2, l97l, proposes todelignify shives and specks which have not been sufficiently delignifiedin the first stage. The first delignification stage is a quite normalone, removing the bulk of lignin. In the second delignification stage,chlorine is added in a large excess, leaving as much as 2 to 10 grams ofactive chlorine per kilogram of pulp unreacted at the end of thetreatment. The high concentration of active chlorine throughout thedelignifying stage is claimed to be a prerequisite for an effectivebleaching of the shives. The addition of chlorine dioxide in the seconddelignification stage is a secondary possibility, used to protect thecellulose in a way that has long been known. Such addition of a smallamount of C10 is a standard procedure in chlorination (see TAPPIMonograph No. 27, pp -136). This addition does not change the stage intosomething other than the normal chlorine stage, performed 'at a somewhathigher concentration of chlorine than usual. In the second stage,moreover, the temperature is held below 35 C., and preferably between 20and 30 C.

In accordance with the instant invention, it has been determined that asecond chlorination stage in which a mixture of chlorine and chlorinedioxide containing an amount of chlorine dioxide within the range fromabout to about 50 percent by weight of the mixture calculated asavailable chlorine is used at a pH within the range from about 1 toabout 3.5 can be arranged to follow as a third or later stage a firstchlorination stage, and a second alkali extraction stage, with goodcontrol of the decomposition of the cellulose pulp, as well as an atleast substantially better delignification than in prior pulp treatmentprocesses, and in many cases, a substantially complete delignification.

The objective of this invention is to remove lignin homogeneouslythroughout the body of cellulose pulp. An insufficient amount ofdelignifying agent is charged in the first delignification stage,leaving a substantial amount of lignin to be delignified in a subsequentstage. The charge in the second delignifying stage is sufficient toremove the remaining lignin, leaving no or only a small excess ofavailable chlorine at the end of the treatment. The bleaching sequenceincludes a chlorinezchlorine dioxide stage, where the percentage ofchlorine dioxide calculated as active chlorine is from about 10 to about50 percent, preferably from 10 to 30 percent. Chlorine dioxide must bepresent. A pure chlorine stage would cause a severe degradation of thecellulose. The higher the temperature, the larger amount of chlorinedioxide that is needed. A high temperature, from 40to 70 C. is used.Such a high temperature range has not earlier been possible inchlorination, due to cellulose degradation. The high temperature hasbeen made possible by adding a sufficient amount of chlorine dioxide.

In the multistage pulp bleaching process of the invention, the firstchlorination stage can be carried out with chlorine, with chlorinedioxide, or with a mixture of chlorine and chlorine dioxide either inthe usual way, as shown in Table A, or preferably, under relatively mildconditions, so that delignification is incomplete. The cellulose pulp isthen subjected to alkali treatment, and then in the third or somesubsequent stage before the bleaching is complete, a continueddelignification stage using a mixture of chlorine and chlorine dioxidein accordance with the invention.

The delignification stage of the invention is carried out underrelatively severe conditions, unlike the preliminary chlorination stage,whether this stage is a normal chlorination, or a mild chlorination. Inthe delignification stage of the invention, the pH is within the rangefrom about 1 to about 3.5. At a pH from 4 to 5 hypochlorous acidconcentration reaches a maximum, and at a pH above 5, especially at from6 to 7, the hypochlorous acid activity is at a maximum. This isundesirable, because of the degradation that results.

The pulp consistency is rather high, and is within the range from about3 to about percent. An ordinary chlorination stage without chlorinedioxide addition is carried out at C. or lower. The temperature in thedelignification stage of the invention is rather high,

within the range from about 40 to about 70 C., and the treatment time iswithin the range from about to about 300 minutes. These temperature andtime conditions are not usually employed in a chlorination step, butcorrespond instead to theconditions of a hypochlorite or chlorinedioxide treatment.

In the delignification stage of the invention, the proportion ofchlorine dioxide in the chlorine-chlorine dioxide mixture can be withinthe range from about 10 to about 50 percent chlorine dioxide, preferablywithin the range from about 10 to about 30 percent chlorine dioxide. Theamount is calculated in terms of the amount of available chlorineequivalent to the chlorine dioxide.

The chlorine is employed as chlorine liquid or-chlorine gas, and isadded to an aqueous suspension of the pulp in any conventional way.Chlorine dioxide is also added as such, or can be formed in situ.Usually, it is preferable to prepare chlorine dioxide as and whenrequired, and then add it directly to the aqueous pulp mixture. Chlorineand chlorine dioxide should preferably be mixed thoroughly before theaddition thereof into the aqueous pulp mixture.

The delignification stage of the invention offers several advantagesover prior treatments, among them a lower cost for the chemicalsrequired, and a very much smaller risk of decomposition of thecellulose. It is possible because of the benefits of the delignificationstage of the invention to lower the cost for the chemicals re quiredduring bleaching, because of a reduction in the amounts of thechemicals, and rearrangement of the order of the various treatmentstages of the bleaching process, with an unchanged or even improvedquality of the pulp, in consequence.

In the process of the invention, the initial chlorination can be theusual chlorination described in Table A. Preferably, however, thechlorination stage is quite incomplete, and the chlorine charge rathersmall. Thus, the chlorine charge can be within the range from about 0.15to about 0.25 percent chlorine per kappa number unit of the unbleachedpulp, so that at the conclusion of the chlorination a small excess,within the range from about 0.1 to about 0.5 percent chlorine, remains.Chlorine dioxide can be substituted in whole or in part for chlorine, inamounts to supply an equivalent amount of chlorine, and an equivalentexcess of chlorine at the conclusion of the process. Thus, mixtures ofchlorine and chlorine dioxide can be used, with like effect. Thetreatment time can be rather small, as low as twentyminutes, up to abouttwo hours. Under these conditions, there is no noticeable decompositionof the cellulose.

The pulp consistency during this incomplete chlorination stage is withinthe usual range from about 1 to about 4%, and the temperature is alsowithin the usual range from about 5 to about 25 C.

An alkali extraction stage is interposed following the chlorine-chlorinedioxide treatment of the invention to remove the alkali soluble ligninderivatives and other alkali-soluble extractives formed in that stage.This alkali extraction can directly follow the Cl -ClO treatment, or beinterposed at a later stage.

The alkali extraction can be under the usual conditions, as shown inTable A. However, lower alkali coricentrations can be used, since alarge proportion of alkali-soluble extractives will have been removed inthe first alkali extraction. Thus, the amount of alkali can be from 0.5to 1.5 percent, and at such amounts the pH will be from 8 to 12.5 at theend of the extraction, as in the usual alkali extraction stage.

In the first alkali extraction, on the other hand, the application afteran incomplete first chlorination impliesa 15 to 25 percent saving in theamount of alkali, in this stage, in comparison to the application aftera conventional chlorination. In the alkali extraction stage followingthe chlorine-chlorine dioxide treatment of the invention, the pulpconsistency is within the usual range from about 4 to about 18 percent,the temperature within the usual range from about 25 to about 30 C., upto about 130 C. in a hot alkali refining stage, and the treatment timecan range from several minutes lulose molecules, and (3) the extinctionat 205 nm, i.e., ultraviolet light (which is determined for a solutionof the pulp in phosphoric acid containing mg. of pulp per ml. and is ameasurement of the quantity of lignin to about 5 hours. 5 remaining inthe pulp). The higher the viscosity, the

FOllowing the chlorination and alkali extraction greater the strength ofthe pulp, and the lower the exstages, the pulp is then subjected to thedelignification tinction, the greater the extent of delignification.stage in accordance with the invention. The pulp at this The followingExamples represent preferred embodipoint may well be less completelydelignified than after ments of the invention. conventional chlorinationand alkali extraction proc- 1Q esses, but nonetheless at the conclusionof the deli a EXAMPLE 1 fication stage of the invention, the resultmgpulp is more delignified and has a better quality. This means unbleachedCellulose P p P p from Pirle that the final bleaching of the deli hifiedpulp can be wood y the sulfate p a ng a appa num er f performed withless chemical consumption and results 5 31 pp number is a measurement ofthe lignin in better quality of the final bleached pulp. Quality ofContent of the p p, and thus of its nsumption of the final bleached pulpis evaluated by the usual critebleachlhg chemlcals- The determination ofPP "P ria: (1) brightness, percent SCAN before and after her accordingto the Standard SCAN-C 1159) was aging at el v t d t r t (2) vi i hi h ichlorinated and treated with alkali as the first two determined for anaqueous copper ethylene diamine Stages of the bleaching p The pulp as thn disolution of the pulp according to SCAN C15;62, d vided into fourbatches, and the bleaching was continwhieh is a measurement of the meanpolymerization ued in a series of steps. The conditions are outlined indegree of the cellulose, i.e., the chain length of the cel-' able lTABLE I BATCH NUMBER Stage Treatment l 2 4 (1)-Chlorination Pulpconsistency 3.5% Temperature 20C. Chlorine charge 6.8% Treatment time 1hour Chlorine residue at end of treatment 0.15%

(2) Alkali Extraction Pulp consistency 8'71 Temperature C. NaOH charge371" Treatment time 2 hours pH at end of treatment 11.6

3) HOCI CHLORINE (210 Pulp consistency 8% 6% 6% 6% Temperature C. 45 4040 NaOCl' cl cl Cl Charge 1.7% l.08% 0.775% 1.08% NaOH Clo, c10 C10,

Charge 0.68% 0.47 0.775 0.47 Treatment time (hours) 2 3 3 3 Chlorineexcess at end of treatment 0.2% 0.067: 0.02 0.01% pH at end of treatmentl0.7 2.1 ll

' (4) Alkali Extraction Pulp consistency None Temperature 50C- NaOHcharge V711 Treatment time (hours) 2 pH at end of treatment 11.5

(5) Chlorine Dioxide Pulp consistency 6% Temperature C.

ClO charge (as equivalent C1,) Treatment time (hours) Surplus chlorineequivalent at end of treatment Table i Continued Stage Treatment BATCHNUMBER (6) Alkali Extraction Pulp Consistency 8% Temperature 50C.

NaOH charge 0.7%

Treatment time (hours) 2 pH at end of treatment ll.2-l 1.3

(7) Chlorine Dioxide Pulp consistency 6% Temperature 85C.

ClO charge (as equivalent C1 0.9%"

Treatment time 5 hours Surplus chlorine equivalent at end of treatment("/2) (LIZ-0.3%

PROPERTIES OF BLEACHED PULPS Brightness, '7: SCAN 93.6 93.7 93.7 93.6

Brightness after ageing l6 hours at 180C, 80% rh 86.8 87.7 87.1 83.2

Brightness after ageing 16 hours at 120C, dry atmosphere 87.7 88.3 88.088.5

Viscosity, cp Tappi 44 55 55 59 Extinction at 205 nm, cm 0.35 0.3l 0.300.36

based on Cl equivalent dry pulp weight Batch 1 served as a control inwhich a conventional hypochlorite treatment was used as stage (3)instead of the chlorine-chlorine dioxide treatment in accordance withthe invention, which was used for batches 2 to 4. It is apparent fromthe properties of the finished pulp that pulps of equivalent or improvedbrightness were obtained using the chlorine-chlorine dioxide treatmentof the invention. These pulps also had a distinctly higher viscosity,and a lower extinction. The viscosity for a sulfate pulp up toviscosities up to approximately 60 cp Tappi shows a direct correlationbetween the strength of paper produced from the pulp, and the magnitudeof viscosity. At levels above 60 cp Tappi, increases in the viscositymay improve strength but not to the same extent.

Data show that replacement of the hypochlorite treatment stage with thedelignification stage of the invention gives an increase in thepotential strength properties of the pulp, and an improveddelignification. Moreover, there is a marked reduction in the chemicalcosts for bleaching in the case of batches 2 to 4, as compared to batch1.

EXAMPLE 2 Using the same starting cellulose pulp as in Example 1, ableaching operation was carried out in accordance with the stagesoutlined in Table 1, except that in the first chlorination stage, 5percent of the chlorine charge was replaced by an equivalent quantity ofchlorine dioxide. The results obtained were similar to those shown inTable I, with the exception that the viscosities were higher:

TABLE II Batch No. l 2 3 4 Viscosity, cp Tappi 64 87 76 83 Because ofthe use of the chlorine dioxide in substitution for chlorine, the costfor the chemicals was somewhat higher than the cost of the processoutlined in Table I.

EXAMPLE 3 process and having a kappa number of 31 was used.

This was divided into six batches, which were bleached in accordancewith the stages outlined in Table III. Batches 1, 2 and 3 were-controls,and batches 4 to 6 illustrate the use of the chlorine-chlorine dioxidedeligni- 5 fication of the invention, as stage (3), in replacement ofthe hypochlorite stage (3) used for batches l to 3.

TABLE III BATCH NUMBER Stage Treatment 1 2 3 4 5 6 (l) Chlorination Pulpconsistency 3.5% 3.5% 3.5%

Temperature 20C. 20C. 20C.

Cl charge 6.8% 6.46% 5.25%

ClO charge 0.34%

Time (hours) I l l/3 Cl excess 0.2% 0.2% 0.1%

' TABLE m mi'iiied BATCH NUM BER Stage Treatment 1 2 3 4 5 6 (2) AlkaliExtraction Pulp consistency 8% 8% Temperature 50C. 50C. NaOH charge 3%2.3% Time (hours) 2 10.9-11.4 pH at end of treatment 10.9-11.4 10.9-11.4

(3) Hypochlorite C1 +C10,

Pulp consistency '871 8% 8% 6% 6% 6% Temperature C. 40 40 40 40 40 40NaOCl NaOCl' NaOCl' C1 C1 C1 Charge 1.08 1.08 2.32 1.94 1.73 1.51

NaOH NaOH NaOH C10 C10 C10,

Charge 0.43 0.43 0.93 0.22 0.43 0.65 Time (hours) 2 2 2 3 3 3 pH at endof treatment 10.6 10.9 109 1.65 1.7 1.85 Cl equivalent at end oftreatment ("/b) 0.05 1 0.10 0.02 0.02 0.02

(4) Alkali Extraction Pulp consistency None 871 Temperature C. NaOHcharge 171 Time (hours) 2 pH at end of treatment l111.2

(5) Chlorine Dioxide Pulp consistency 6% 6% Temperature C.

C10 charge (as equivalent C1 1% of dry pulp weight Time (hours) 3Surplus chlorine equivalent at end of treatment. (1.05 to (1.2071 01 drypulp weight (6) Alkali Extraction Pulp consistency 87! Temperature 50C.

NaOH charge 0.771 of dry pulp weight Time (hours) 2 pH at end oftreatment 11.2-11.3

(7) Chlorine Dioxide Pulp consistency 6% Temperature C.

C10 charge (as equivalent C1,) 0.971 of dry pulp weight Time (hours) 5Surplus chlorine equivalent at end of treatment 0.2-0.37r dry pulpweight Bleached Pulp Properties Brightness. 71 SCAN 91.9 91.7 93.0 92.992.4 92.2 Ditto after ageing for 16 hrs/80C.. 809? rh 85.2 84.9 84.585.4 85.3 84.8 Ditto after ageing l6 hrs/120C.. dry atmosphere 85.4 85.185.1 86.9 86.2 85.6 Viscosity. cP Tappi 56 89 68 86 90 93 Extinction at205 nm. em" 0.38 0.40 0.53 0.36 0.36 0.35

based on Cl. equivalent 1 dry pulp weight It is evident from the datathat the cost for the chemicompared t bat he l to 3. Batches 4, 5 and 6gave as leals tn the treatment of batches 4 t0 6 as reduced, and high aviscosity as in the case of batch 2, in which the in pite of hi n imprrigh was obtained, as bleaching was conducted with the addition ofchlorine dioxide in the chlorination stage. Moreover, the finaldelignification is more complete, as indicated by the extinction valuesfor batches 4 to 6.

EXAMPLE 4 This Example shows that if a delignification stageinaccordance with the invention is employed, delignification prior tofinal bleaching can be carried further than in a conventional bleachingprocess, without the TABLE IV BATCH NUMBER Stage Treatment I l 2 3 4 (l)Chlorination Pulp consistency 3.5% 3.5% 3.5% 3.57: Temperature 20C. 20C.20C. 20C. 71 Cl 7.22 7.22 5.6 5.6 71 Cl 0.38 0.38 0.30 0.30 Time 1 hourI hour min. 20 min. Clexcess at end of treatment 0.3 0.3 0.1 0.1

(2) Alkali Extraction Pulp consistency 8'71 Temperature 50C. "/1 NaOH2.8% Time 2 hours pH at end of treatment ll.ll 1.5

(3) Hypoehlorite Cl -ClO Pulp consistency 8% 8% 6% 6% Temperature C.40C. C. 50C. Charge NaOCl NaOCl Cl Cl 1.9% 1.9% 2.38% 2.38% Charge NaOHNaOH ClO C10.

0.73% 0.73% 0.42% 0.42% Time (hours) 2 2 3 3 Cl excess at end oftreatment 0.43 0.43 0.23% 0.23% pH at end of treatment H0 11.0 1.7 l 7(4) Alkali Extraction Pulp consistency None 8% 8% Temperature 50C. 50C.72 NaOH l% 1% Time 2 Hours 2 Hours pH at end of treatment l0.95 10.95

(5) CIO Pulp consistency 8% None 8% 8% TemperatureC. 70 85 C10, 1.3712.0% l.3'7r 2.0% Time 3 hours 5 hours 3 hours 5 hours Cl equivalent atend of treatment (7r) 0.6 0.07 0.20 0.l6

(6) Alkali Extraction Pul consistency 8% I None 8% None Temperature 50C.50 "X'NaOH 0.7% 0.7% Time 2 hours 2 hours pH at end of treatment TABLEIV Continued BATCH NUMBER Stage Treatment 1 2 3 4 Pulp consistency 671None 6% None Temperature 85C. 85C. C10, (1.97: 0.9% Time hours 5 hoursC1 equivalent at end of treatment ("/t) (1.13-0.14 0.13-0.14

Properties of Bleached Pulp Brightness according to SCAN, '71 93.3 90.694.1 92.1 Brightness after ageing for 16 hours at 80C., 11071 86.3 83.586.9 85.4 Brightness aftcr ageing for 16 hours at 120C, dry atmosphere87.3 83.4 89.0 85.2 Viscosity according to Tappi cP 61 S0 65 61Extinction at 205 nm, em 0.361 0.481 0.293 0.364

' based on C1. equivalent 2 dry pulp weigh\ It is apparent from the datathat batch 2, delignified according to the conventional procedure usinghypochlorite, and batch 4, delignified in accordance with the presentinvention, which were finally bleached using only one chlorine dioxidestage, showed a lower degree of brightness than corresponding pulpsfinally bleached with two chlorine dioxide stages, batches 1 and 3,respectively. The brightness of the pulp batch 4 delignified inaccordance with the invention lay at an acceptable level, and theviscosity is as high as that of batch 1, which was bleached with twochlorine dioxide stages in the final bleaching stage, but withconventional delignification including a hypochlorite stage.

Delignification according to the invention followed by final bleaching,using only one chlorine dioxide stage, can be considered as amodification of the generally used sequency C E D E D. The inventionthus provides a reduced cost of chemicals, and gives good pulp qualitywith regard to brightness and mechanical strength. Also, when using theprocess of the invention,

- it is possible to carry the delignification further with processcomprising in a first chlorination and delignifi-.

cation stage chlorinating cellulose pulp with chlorine, with chlorinedioxide, or with a mixture of chlorine and chlorine dioxide underrelatively mild conditions, so that delignification is incomplete; in'analkali treatment stage alkali-treating the chlorinated pulp; and then,as a third or subsequent stage,'before the bleaching is complete, in asecond chlorination and delignification stage, chlorinatingthe alkalitreated pulp with a mixture of chlorine and chlorine'dioxide comprisingfrom about percent to about 'percent chlorinedioxide 3. A process inaccordance with claim 1, which comprises alkali treating the chlorinatedpulp subsequent to the second chlorination and delignification stage.

4. A process in accordance with claim 1, which comprises alkali treatingthe delignified pulp and then treating the alkali treated delignifiedpulp with a member selected from group consisting of hypochlorite andchlorine dioxide.

5. A process in accordance with claim 1, in which the first chlorinationand delignification is effected at a pulp consistency within the rangefrom about 1 to about 4 percent, a temperature from about 5 to about 25C. for from about 1/3 to about 2 hours, with sufficient chlorine,supplied as chlorine, chlorine dioxide, or both, to provide from about0.1 to about0.5 percent excess chlorine at the end of the chlorination.

6. A process in accordance with claim 1, in which the alkali treatmentis effected at a pulp consistency within the range from about 4 to about18 percent at a temperature from about 25 to about 130 C. for a periodof time up to about 5 hours, and with sufficient alkali to afford a pHfrom about 8 to about 12.5 at the end of the alkali treatment. V

7. A process in accordance with claim 1, in which the proportion ofchlorine dioxide in the chlorine-chlorine dioxide mixture in the secondchlorination and delignification stage is within the range from about 10to about 30 percent chlorine dioxide, calculated as the amount ofchlorine equivalent to the chlorine dioxide. 4 i l LII Page 1 of 4UNITED STATES PATENT OFFICE (IERHFICATE 6F CORRECTION Patent NO. 3, 2,Dated pri 19 4 Inventods) lngemar Lise-Albin Croon et 21.

It is certified that error appears in the above-identified patent a andthat said Letters Patent are hereby corrected as shown below:

Page 1, [75] line 2 after "Salgvagen" insert Domsjoverken line 3 v after"Strandvagen" insert -Hallstavik Column 4, line 43 after"delignification" please insert a period Column 7, line 6 "FOllowing"should be Following-- Q i l -l .Y

Page 2 of 2 UNITED STATES PA'IENT OFFICE @E'llFlCATE 9F CORRECTIONPatent No. 958 Dated April 9, 1974 lngemar Liss A1b1n Croon et 211.

It is certified that error appears in the aboveidentified patent andthat said Letters Patent are hereby corrected as shown below:

Column 8, Table I, heading (3) Batches l, 2, 3 and 4 should be 1 (3)HOG]; CHLORINE C10 Pulp consis- 8% 6% 6% 670 tency Temperature 45 40 4060 Naomi d2 d2, ge 1.7% r 1.08% 0.775% rod-5 NaOH (1102 153 01o Charge0.68% 0.47 0.775 0.47 Treatment 2 3 3 3 time (hours) Chlorine 0.2%0.069% 0.027% 0.0175

excess at end of treatment pH at end of 10.7 2.1 2.15 2.1

treatment Page 3 of 3 UNITED STATES PATENT OFFICE CERTTFICATE OFCORRECTION Patent No 3, 802, 958 Dated Amf'fl 9 1974 Inventor(s) IngemarLiss Albin Croon et 211.

It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Column 8, Table I, heading 4, Batch 3 Page 4 of 4 UNITED STATES PATENTOFFICE CER'IIFICATE OF CORREC'UQN Q Patent X0. 3, 802,958 Dated April 9,1974:

Invent r(5) Ingemar Liss-Albin Croon et al It is certified that errorappears in the above-identified patent and that said Letters Patent arehereby corrected as shown below:

Column 14, Table IV, heading Batch 2 i should be 0 Pulp consis- 8% 9,6 8875 tency Temperature 7O 85 c10 1.3% 2.0% 1.3% 2.09% Time 2 3 hours 5hours 3 hours 5 hours C1 equivalenfi' 0.6 0.07 0,20 0.16 .5 at end oftreatment (,0)

Column 15, line 40 "sequency" should be --sequence- 0 ,Zn'gncd andScaled thisthirtieth D f March 1976 [SEAL] Attest: O

RUTH C. MASON C. MARSHALL DANN Atlesling Officer Commissioner ofPatentsand Trademarks

2. A process in accordance with claim 1, which comprises chlorinatingthe pulp in the first chlorination and delignification stage withchlorine.
 3. A process in accordance with claim 1, which comprisesalkali treating the chlorinated pulp subsequent to the secondchlorination and delignification stage.
 4. A process in accordance withclaim 1, which comprises alkali treating the delignified pulp and thentreating the alkali treated delignified pulp with a member selected fromgroup consisting of hypochlorite and chlorine dioxide.
 5. A process inaccordance with claim 1, in which the first chlorination anddelignification is effected at a pulp consistency within the range fromabout 1 to about 4 percent, a temperature from about 5* to about 25* C.for from about 1/3 to about 2 hours, with sufficient chlorine, sUppliedas chlorine, chlorine dioxide, or both, to provide from about 0.1 toabout 0.5 percent excess chlorine at the end of the chlorination.
 6. Aprocess in accordance with claim 1, in which the alkali treatment iseffected at a pulp consistency within the range from about 4 to about 18percent at a temperature from about 25* to about 130* C. for a period oftime up to about 5 hours, and with sufficient alkali to afford a pH fromabout 8 to about 12.5 at the end of the alkali treatment.
 7. A processin accordance with claim 1, in which the proportion of chlorine dioxidein the chlorine-chlorine dioxide mixture in the second chlorination anddelignification stage is within the range from about 10 to about 30percent chlorine dioxide, calculated as the amount of chlorineequivalent to the chlorine dioxide.